renderer: improve thread safety

Since we share render backends between the viewer and cache tasks now, we
should ensure the render backend is thread safe.

app/task/cache/cache.cpp

@@ -57,7 +57,7 @@ bool CacheTask::Run()
     }
   }
 
-  Render(video_range, audio_range, QMatrix4x4(), true);
+  Render(video_range, audio_range, true);
 
   download_threads_.waitForDone();
 

app/task/export/export.cpp

@@ -64,17 +64,17 @@ bool ExportTask::Run()
 
   frame_time_ = Timecode::time_to_timestamp(range.in(), viewer()->video_params().time_base());
 
-  QMatrix4x4 mat;
-
   if (params_.video_enabled()) {
 
     // If a transformation matrix is applied to this video, create it here
     if (params_.video_scaling_method() != ExportParams::kStretch) {
-      mat = ExportParams::GenerateMatrix(params_.video_scaling_method(),
-                                         viewer()->video_params().width(),
-                                         viewer()->video_params().height(),
-                                         params_.video_params().width(),
-                                         params_.video_params().height());
+      QMatrix4x4 mat = ExportParams::GenerateMatrix(params_.video_scaling_method(),
+                                                    viewer()->video_params().width(),
+                                                    viewer()->video_params().height(),
+                                                    params_.video_params().width(),
+                                                    params_.video_params().height());
+
+      backend()->SetVideoDownloadMatrix(mat);
     }
 
     // Create color processor
@@ -99,7 +99,7 @@ bool ExportTask::Run()
     audio_data_.SetLength(range.length());
   }
 
-  Render(video_range, audio_range, mat, false);
+  Render(video_range, audio_range, false);
 
   bool success = true;
 

app/task/render/render.cpp

@@ -78,11 +78,8 @@ struct HashDownloadFuturePair {
 
 void RenderTask::Render(const TimeRangeList& video_range,
                         const TimeRangeList &audio_range,
-                        const QMatrix4x4& mat,
                         bool use_disk_cache)
 {
-  backend_->SetVideoDownloadMatrix(mat);
-
   double progress_counter = 0;
   double total_length = 0;
   double video_frame_sz = video_params().time_base().toDouble();
@@ -245,8 +242,10 @@ void RenderTask::Render(const TimeRangeList& video_range,
     }
   }
 
-  // `Close` will block until all jobs are done making a safe deletion
-  backend_->Close();
+  if (backend_is_ours_) {
+    // `Close` will block until all jobs are done making a safe deletion
+    backend_->Close();
+  }
 }
 
 void RenderTask::SetAnchorPoint(const rational &r)

app/task/render/render.h

@@ -40,7 +40,6 @@ public:
 protected:
   void Render(const TimeRangeList &video_range,
               const TimeRangeList &audio_range,
-              const QMatrix4x4 &mat,
               bool use_disk_cache);
 
   virtual QFuture<void> DownloadFrame(FramePtr frame, const QByteArray &hash) = 0;